Apparatus for making geophysical explorations



Feb. 29, 1944.

K. H. EVJEN ETAL APPARATUS FOR MAKING GEOPHYSICAL EXPLORATIONS Filed Jan. 8, 1942 REcT.

Patented Feb. 29, 1944 UNITED STATES PATENT OFFICE l APPARATUSl FOR MAKING GEOPHYSICAL EXPLORATION S Application January 8, 1942, Serial No. 425,993

3 Claims.

This invention relates to a method of electrical prospecting and to an apparatus for carrying out the same and isranimprovement on the system disclosed in a patent to'Knute Evjen, No. 2,294, 395, dated Sept. 1, 1942, which embodies an earth current circuit including a current source and a controlled reversing means for passing a current reversed at a predetermined controlled low frequency through the earth between spaced earth electrodes, and a potential circuit including potential pick-up electrodes located at spaced points adjacent the earths surface and lying within the field of influence of the earth current. The potential circuit is provided with reversing means synchronized with the first reversing means for converting the picked-up potential into a unidirectional potential suitable for actuating a direct current measuring instrument such as a galvanometer. The earth current circuit and the ptential circuit are coupled -by a coupling circuit which includes means for producing a potential drop proportional to the total current flowing in the earth current circuit and for balancing a controllable portion of this -potential drop against the picked-up potential so as to obtain, by a single reading, a measurement which indicates directly the earths apparent resistivity.

An object of this invention is to provide a novel and improved system of the type above indicated.

Another object is to provide a novel and improved coupling circuit in a system of the above type.

Various other objects and advantages will be apparent as the nature of the invention is more fully disclosed.

Although the novel features which are believed to be characteristic of this invention are more particularly pointed out in the claims appended hereto, the invention itself may be better understood by referring to the following description, taken in connection with the accompanying drawing, in which certain specific embodiments thereof have been set forth for purposes of illustration.

In the drawing:

The figure is a schematic representation of an electrical prospecting system embodying the present invention.

Referring to the drawing, the earth current circuit comprises a source I0 of direct current, such as a generator or battery. The source I0 is connected in series circuit with a line II, a resistance I2, an ammeter I3 and aline I4. Lines II and I4 are connected to segments l5 and I6 respectively of a commutator Il which is driven by suitable means to be described. Alternating current is taken from the. commutator I'I by means of lines I8 and I9 respectively which are connected to suitable ground electrodes 2l and 2l herein referred to for convenience as current electrodes. The current electrodes 20 and 2I are spaced a convenient distance apart, which may be of the order of several thousand feet, and when the commutator I1 is driven at a suitable speed, an alternating current having a frequency corresponding to the speed of rotation of the commutator, passes through the earth circuit. This current may be measured by the ammeter I3 and may be adjusted within desired limits by suitable regulation of the source I0.

It will be noted that a potential drop is built up across the resistance I2 which is directly proportional to the current flowing in the earth circuit. This potential drop is utilized in the present system for obtaining the measuring potential. For this purpose, an interrupter 25 is connected across the resistance I2 and the opposite ends of said resistance are connected through condensers 26 to the primary 21 of the transformer 28. The interrupter 25 periodically short circuits the resistance I2 and thereby produces a variable potential across the .primary of the transformer 21 which produces an alternating current in the secondary 29 thereof. The interrupter 25 may operate at any desired frequency, dependent upon the characteristics of the transformer 28. A frequency of the order of 500 per second has been found suitable, although it is to be understood that various other frequencies may be employed. The condensers 26 should be of sufliciently high capacity to eiiciently pass currents of this frequency or may be omitted in certain instances. In the present embodiment, the condensers pre- Vent the direct current from passing through the primary of the transformer and saturating the magnetic circuit thereof. They also prevent a direct metallic connection between the transformer and the earth current circuits.

The alternating current induced in the secondary 29 is rectified by a suitable rectier 30 which may be of any desired type. such, for example, as a space discharge rectifier or a chemical rectier, such as a copper oxide rectifier. The direct current output of the rectifier 30 is connected by lines 38 to brushes 10 and II which engage respectively segments 'l2 and 'I3 of a commutator 14. Reversing contacts '15a and 76a of the commutator I4 are connected by lines i5 and 'I6 to a potentiometer 'I8 having adjustable taps 'i8 and B0. The tap is connected by a line 8| to a potential pick-up electrode 5I. The tap 'I9 is connected by a line 82 to a segment 45 of a commutator 46. The segment 4B of the cominuon a shaft Sii which is driven, through a suitableA gear train indicated at 62, by a motor Si having a speed control device 83 and an indicator 64 to indicate the speed of operation thereof.

The galvanometer 43 is preferably of the highly damped type so that it is influenced only by the direct current component and not by any ripple or other alternating current component in the circuit under measurement.

Operation In the operation of this system. the current electrodes 20 and 2| are located at selected points in the earth and the potential electrodes 5I and 51 are located at points within the eld of iniluence of the earth currents produced between the current electrodes. The source I is adjusted to pass a predetermined amount of current through the earth circuit, as indicated by the ammeter I3. The frequency of alternation of the current is regulated by making suitable adjustment of the control handle B3. If the measurements are to be taken with direct current, the commutator I1 remains stationary. For low frequency alternating current, the commutator is slowly rotated at the selected speeds as indicated by the speed indicator 66. It will be noted that the potential drop in the resistance l2 will vary in proportion to the amount of current flowing in the earth circuit. The interrupter 25, by intermittently bridging this resistance i2, causes a pulsating current to flow in the primary of the transformer 28 which bears a deiinite relationship to the amount of current owing in the earth circuit. The value of the resistance i2 is so chosen that only'a small fraction of the total current owing in the earth circuit is converted into pulsating current. It may have a value, for example, of from 1 to 20 ohms. The pulsating current thus produced may, for example, correspond to about 1% of the earth current. The alternating current in the secondary of the transformer 28 is rectified by the rectier 3@ to produce a direct current.

The direct current derived from the rectifier is reversed by the commutator 1t to produce in the potentiometer 18 a potential drop which is reversed in synchronism with the earth current and, consequently, in synchronism with the p0- tential picked up by the pick-up electrodes -i and 51 and is applied to the potential circuit in a direction` to oppose the picked-up potential.

The potential derived from the potential pickup electrodes I and 51 is balanced against the potential taken from the potentiometer 18, is rectified by the commutator 86, and applied as a direct current to the galvanomcter 43.

,In certain instances, suitable lters may be incorporated in the rectier circuit to eliminate the ripple and produce a uniform direct current, or the same eect may be obtained by using a highly damped galvanometer 43. Inasmuch as there is no direct ohmic connection between pos tentiometer 1B and the earth current circuit ex cept thi/rough the earth itself. the potential developed across the potentiometer may be used to measure the potential diiference existing between the potential electrodes 5| and 51.

With the circuit operating in this manner, suitable adjustments are made of the contacts 19 and 80 to produce a zero reading on the galvanometer 43 which indicates that the average voltage drop across the intermediate portion of the potentiometer 18 between the contacts 19 and 80 exactly balances the potential which is picked up by the earth potential electrodes 5| and 5l.

The eiect of any residual earth current which would tend to produce a constant direct potential across the electrodes 5I and 51 is eliminated by suitable adjustment of the potentiometer 54. This adjustment may be made, for example, while no current is flowing through the earth circuit so that the only effect on the galvanometer 43 represents the effect of the residual earth current. Since the earth current impresses a direct current potential across' the probe (or potential) electrodes, any unbalance between this potential and the potential taken from potentiometer 5A, will appear as an Aalternating potential after commutation by commutator 46. This will make the galvanometer'kick from one side to the other in synchronism with the commutation. Any drift in the natural earth potential may thus be immediately perceived, and can be compensated by readjusting potentiometer 5d until the picking of the galvanometer is eliminated.

l.A lter may be included in the measuring circuit if desired in order to eliminate ripples or uctuations from various causes such, for example, as those produced by the action of the commutatore.

The potentiometer 18 may be suitably graduated in convenient units and when the balance is obtained, the reading corresponding to the point of adjustment is noted. The frequency is also noted from the indicator 66. The potential developed across the potentiometer 18, being dependent upon the total value of the earth current at the instant that the measurement is being taken, automatically compensates for any variations in electrode resistance or in contact resistance and eliminates any necessity for making simultaneous measurements of the earth current and the induced potential.

This measurement isrepeated with different frequencies untila complete potential spectrum of the area between electrodes 5I and 51 is obtained. These electrodes may then be moved to another location and the measurements repeated to obtain a potential spectrum at various frequencies in the manner above' pointed out. The

'f various spectra thus obtained may be interpreted to show the characteristics of the earths strata in the area under investigation. It ls to be understood, of course, that the potential electrodes may be variously positioned with respect to thecurrent electrodes 2U and 2i and that the positions thereof may be changed in accordance with the depth which is under investigation at any particular instant.

It will be noted that in the above described system only a single reading need be taken and the necessity for mathematical calculations is avoided. The system is accordingly well adapted to eld use andmay be operated by unskilled persons. The data thus compiled may be interpreted by geological physicists in accordance with well known principles.

The size of the electrodes may vary over wide limits. In practice, metal stakes about one or two inches in diameter and 3 ft. or 4 ft. in length have been found satisfactory. The distance between the current electrodes may vary widely, for example, from 200 ft. to 5000 ft., depending upon the depth to be investigated. The potential of the source I may likewise be adjusted as desired. Voltages of 100 volts to 200 volts and earth currents of one to two amperes have been found satisfactory.

Although certain specific embodiments of the invention have been shown for purposes of illustration, it is to be understood that the invention is not to be limited thereto but that various changes and modifications may be made therein as will readily appear to a person skilled in the art. The invention is only to be limited in accordance-with the scope of the following claims.

What is claimed is:

1. A system for electrical prospecting, comprising an earth current circuit including a source of direct current, a pair of spaced earth electrodes, and reversing means connected between said source and said electrodes for reversing the direct current at a predetermined controlled frequency whereby an earth current having a predetermined frequency is passed through the earth between said electrodes, a potential circuit including potential pick-up electrodes located at spaced points lying within the eld of iniiuence of said earth current, a direct current indicating device connected to indicate the potential of said potential circuit, and reversing means synchronized with said rst reversing means and connected between said' potential pick-up electrodes and said indicating device t0 convert the picked-up potential difference into a uni-directional potential suited to actuate said indicating device, and coupling means connected between said earth current circuit and said potential circuit to impress on said potential circuit a balancing potential which is directly proportional to the current flowing in said earth current circuit at the instant of measurement, said coupling means comprising' a resistance connected in series with said earth current circuit on the direct current side of said iirst reversing means to carry a predetermined'portion of the current in said earth current circuit and arranged to develop a potential drop proportional to the current owing in said earth current circuit, a second resistor connected in said potential circuit on the alternating current side of said second reversing means, an interrupter connected to produce from said potentialdrop an alternating current, a transformer having a primary connected to receive said alternating current, a rectier connected to rectify the current induced in the secondary of said transformer, and a reversing means synchronized with said rst reversing means connected to reverse said rectified current and to supply the same to said second resistor, whereby the potential drop in said second resistor corresponds in direction to the potential picked-up by said potential pick-up electrodes and is directly related to the total current flowing in the earth current circuit at the instant of measurement.

2. The system set forth in claim 1 in which the three reversing means comprise commutators and driving means is provided for driving said commutators in synchronism.

3. The system set forth in claim 1 in which the three reversing means comprise commutators and drivingmeans is provided for driving said commutators in synchronism, said driving means being adjustable in speed to vary the frequency of said commutators.

KNUI'E H. EVJEN. HAAKON M. EVJEN. 

